package least_loaded

import (
	"math/rand"
	"sync"
	"sync/atomic"
	"time"

	"google.golang.org/grpc/balancer"
	"google.golang.org/grpc/balancer/base"
	"google.golang.org/grpc/grpclog"
	_ "google.golang.org/grpc/health"
)

// Name is the name of least loaded balancer.
const (
	Name = "least_loaded"
)

// newBuilder creates a new balance builder.
func newBuilder() balancer.Builder {
	return base.NewBalancerBuilderV2(Name, &PickerBuilder{}, base.Config{HealthCheck: true})
}

func init() {
	balancer.Register(newBuilder())
}

type PickerBuilder struct{}

func (*PickerBuilder) Build(info base.PickerBuildInfo) balancer.V2Picker {
	grpclog.Infof("p2cPickerBuilder: newPicker called with readySCs: %v", info.ReadySCs)
	if len(info.ReadySCs) == 0 {
		return base.NewErrPickerV2(balancer.ErrNoSubConnAvailable)
	}

	var leastLoaded = newLeastLoaded()
	for sc, _ := range info.ReadySCs {
		leastLoaded.Add(sc)
	}

	rp := &Picker{
		leastLoaded: leastLoaded,
	}
	return rp
}

type leastLoadedNode struct {
	item     interface{}
	inflight int64
}

type leastLoaded struct {
	items []*leastLoadedNode
	mu    sync.Mutex
	rand  *rand.Rand
}

func newLeastLoaded() *leastLoaded {
	return &leastLoaded{
		items: make([]*leastLoadedNode, 0),
		rand:  rand.New(rand.NewSource(time.Now().Unix())),
	}
}

func (p *leastLoaded) Add(item interface{}) {
	p.items = append(p.items, &leastLoadedNode{item: item})
}

func (p *leastLoaded) Next() (interface{}, func(balancer.DoneInfo)) {
	var sc, backsc *leastLoadedNode

	switch len(p.items) {
	case 0:
		return nil, func(balancer.DoneInfo) {}
	case 1:
		sc = p.items[0]
	default:
		// rand needs lock
		p.mu.Lock()
		a := p.rand.Intn(len(p.items))
		b := p.rand.Intn(len(p.items) - 1)
		p.mu.Unlock()

		if b >= a {
			b = b + 1
		}
		sc, backsc = p.items[a], p.items[b]

		// choose the least loaded item based on inflight
		if sc.inflight > backsc.inflight {
			sc, backsc = backsc, sc
		}
	}

	atomic.AddInt64(&sc.inflight, 1)

	return sc.item, func(balancer.DoneInfo) {
		atomic.AddInt64(&sc.inflight, -1)
	}
}

type Picker struct {
	leastLoaded *leastLoaded
}

// Pick ...
func (p *Picker) Pick(info balancer.PickInfo) (result balancer.PickResult, err error) {
	item, done := p.leastLoaded.Next()
	if item == nil {
		err = balancer.ErrNoSubConnAvailable
		return
	}
	result.SubConn = item.(balancer.SubConn)
	result.Done = done
	return
}
